In nanotube growth, errors are not an option

Jun 18, 2012 BY MIKE WILLIAMS
In nanotube growth, errors are not an option
Defects in nanotubes heal very quickly in a very small zone at or near the iron catalyst before they ever get into the tube wall, according to calculations by theoretical physicists at Rice University, Hong Kong Polytechnic University and Tsinghua University. Courtesy of Feng Ding/Rice/Hong Kong Polytechnic

(Phys.org) -- At the right temperature, with the right catalyst, there’s no reason a perfect single-walled carbon nanotube 50,000 times thinner than a human hair can’t be grown a meter long.

That calculation is one result of a study by collaborators at Rice, Hong Kong Polytechnic and Tsinghua universities who explored the self-healing mechanism that could make such extraordinary growth possible. That’s important to scientists who see high-quality carbon nanotubes as critical to advanced materials and, if they can be woven into long cables, power distribution over the grid of the future.

The report published online by Physical Review Letters is by Rice theoretical physicist Boris Yakobson; Feng Ding, an adjunct assistant professor at Rice and an assistant professor at Hong Kong Polytechnic; lead author Qinghong Yuan, a postdoctoral researcher at Hong Kong Polytechnic; and Zhiping Xu, a professor of engineering mechanics at Tsinghua and former postdoctoral researcher at Rice.

They determined that iron is the best and quickest among common catalysts at healing topological defects – rings with too many or too few atoms – that inevitably bubble up during the formation of nanotubes and affect their valuable electronic and physical properties. The right combination of factors, primarily temperature, leads to kinetic healing in which carbon atoms gone astray are redirected to form the energetically favorable hexagons that make up nanotubes and their flat cousin, graphene. The team employed density functional theory to analyze the energies necessary for the transformation.

“It is surprising that the healing of all potential defects — pentagons, heptagons and their pairs — during growth is quite easy,” said Ding, who was a research scientist in Yakobson’s Rice lab from 2005 to 2009. “Only less than one-10 billionth may survive an optimum condition of growth. The rate of defect healing is amazing. If we take hexagons as good guys and others as bad guys, there would be only one bad guy on Earth.”

The energies associated with each carbon atom determine how it finds its place in the chicken-wire-like form of a nanotube, said Yakobson, Rice’s Karl F. Hasselmann Chair in Engineering and a professor of materials science and mechanical engineering and of chemistry. But there has been a long debate among scientists over what actually happens at the interface between the catalyst and a growing tube.

“There have been two hypotheses,” Yakobson said. “A popular one was that defects are being created quite frequently and get into the wall of the tube, but then later they anneal. There’s some kind of fixing process. Another hypothesis is that they basically don’t form at all, which sounds quite unreasonable.

“This was all just talk; there was no quantitative analysis. And that’s where this work makes an important contribution. It evaluates quantitatively, based on state-of-the-art computations, specifically how fast this annealing can take place, depending on location,” he said.

A nanotube grows in a furnace as carbon atoms are added, one by one, at the catalyst. It’s like building the peak of a skyscraper first and adding bricks to the bottom. But because those bricks are being added at a furious rate – millions in a matter of minutes – mistakes can happen, altering the structure.

In theory, if one ring has five or seven atoms instead of six, it would skew the way all subsequent atoms in the chain orient themselves; an isolated pentagon would turn the nanotube into a cone, and a heptagon would turn it into a horn, Yakobson said.

But calculations also showed such isolated defects cannot exist in a nanotube wall; they would always appear in 5/7 pairs. That makes a quick fix easier: If one atom can be prompted to move from the heptagon to the pentagon, both rings come up sixes.

The researchers found that very transition happens best when carbon nanotubes are grown at temperatures around 930 kelvins (1,214 degrees Fahrenheit). That is the optimum for healing with an iron catalyst, which the researchers found has the lowest energy barrier and reaction energy among the three common catalysts considered, including nickel and cobalt.

Once a 5/7 forms at the interface between the catalyst and the growing nanotube, healing must happen very quickly. The further new atoms push the defect into the nanotube wall, the less likely it is to be healed, they determined; more than four atoms away from the catalyst, the defect is locked in.

Tight control of the conditions under which grow can help them self-correct on the fly. Errors in atom placement are caught and fixed in a fraction of a millisecond, before they become part of the nanotube wall.

The researchers also determined through simulations that the slower the growth, the longer a perfect nanotube could be. A nanotube growing about 1 micrometer a second at 700 kelvins could potentially reach the meter milestone, they found.

Explore further: Thinnest feasible nano-membrane produced

More information: prl.aps.org/abstract/PRL/v108/i24/e245505

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User comments : 19

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CapitalismPrevails
3.1 / 5 (7) Jun 18, 2012
This is why you have to love computational material science.
jibbles
5 / 5 (4) Jun 18, 2012
space elevator, here we come!
kris2lee
3 / 5 (2) Jun 18, 2012
A nanotube growing about 1 micrometer a second at 700 kelvins could potentially reach the meter milestone.


One meter in one month.
atomsk
2.3 / 5 (3) Jun 18, 2012
One meter in one month.


My calculator says one meter in about 12 days.
TkClick
not rated yet Jun 18, 2012
At the right temperature, with the right catalyst, theres no reason a perfect single-walled carbon nanotube 50,000 times thinner than a human hair cant be grown a meter long
The carbon nanotubes rotate during their growth - so I presume, the limiting factor will be just here.
Hoodoo
5 / 5 (2) Jun 18, 2012
One meter in one month.


You just blew my mind, so I thought I'd do some more calculations.

1 million micrometers in a meter. 1 million seconds is 11.574 days. (not a month, but still pretty slow going)

Now that space elevator cable has to go to geosynchronous orbit (actually it has to much further for the counterweight)

Geosync. is at 35,800 kilometers = 35,800,000 meters
35,800,000 times 11.574 days is 414,349,200 days
That's 1,134,451 years.

OVER A MILLION YEARS FOR A SINGLE NANOTUBE TO GROW TO GEOSYNC. ORBIT LENGTH.

Now obviously they're going to be growing billions of meter-length nanotubes simultaneously and knitting them together for any space elevator, but holy crap-holey that is a serious amount of time.
El_Nose
5 / 5 (3) Jun 18, 2012
you beat me to it Kris -- and then you did the math wrong

Given:
1,000,000 micrometers = 1 meter
1 micrometer / per second

How long to make one meter, in a time frame higher than seconds:
1,000,000 seconds = 1 meter
(1,000,000 sec ) * ( 1 minute / 60 seconds ) * ( 1 hour / 60 minutes ) * ( 1 day / 24 hours )

== 11.574 days

Given:
assuming you can build a facility that creates 1000000 such stands simultaneously so it's thick enough to be a cable ... and you could build 1000 -wow a thousand, yeah right but let's continue - such facilities so that you produce in effect 1 km every twelve days --- how long to build a cable for a space elevator??

36000km to counter balance point
11 days per km

36000 * 11 = 396000 days -- or 1085 years.

This will to get us a space elevator -- we would need to grow perfect tubes at a rate of at least 100x faster to make it practical.
Feldagast
not rated yet Jun 18, 2012
Hoodoo, look at what we did during the space race, with slide rules and brain sweat. We threw a lot of money, time and peoples hard work and dedication at getting in to space and if what we got was worth it, you could say it all paid off. If there was some impending reason to build such a thing we could do it. The great thing about the U.S. is, once they get going they can really produce in massive quantities. Look at WW2 industrial production, conversion of companies to war production, everyone pitching in to produce war materials. I don't know if we could replicate that today, but I would like to think that we could if there was a reason for it.
Husky
not rated yet Jun 18, 2012
El nose, that logistics is some sobering perspective, even if we get all the tec right...maybe the hydrogen gasgun for raw materials and space-x reusable rockets for manned stuff are more realistic path. Would see a nearby future though for shorter electrodynamic spacetethers containing at least a composite of nanotubes and resin.
antialias_physorg
3.7 / 5 (3) Jun 18, 2012
assuming you can build a facility that creates 1000000 such stands simultaneously

Which is a very, very low estimate. Carbon nanotubes have tiny diameters. Growing them by the millions in parallel takes next to no space at all. You could lay out a million next to each other in a shoe box. A billion of them next to each other would take up a meter width (average single wall nanotube diameter is a nanometer).

A solid volume of just 1 cubic meter of nanotubes would contain, laid end-to-end, more than a quadrillion kilometers of nanotubes (10 to the 15th kilometers - which is about 100 light years worth...either I just miscalculated three times in a row or it really is that much).

So depending on the strength of cable we actually need for this I'm not overly pessismistic about the ability to eventually get significant lengths of this stuff made.

Husky
not rated yet Jun 18, 2012
I guess it would all hinge on if you can get Eric Drexlers selfreplicating nanoassemblers working.
kris2lee
3.7 / 5 (3) Jun 18, 2012
you beat me to it Kris -- and then you did the math wrong


Indeed, I'm sorry.

Husky
not rated yet Jun 18, 2012
i have sort of an idea for a mass producing device, you have this centrifuge that you fill with carbon gas, in the walls there will be millions of nanosized holes of slightly larger diameter than a nanotube.

After a while, the long nanotubes will be pulled out contaiing the hot tungsten nanowire, this will be left to cool so that the tungsten wire shrinks and you can pull it out the nanotube. The isn't probably much room/margin for small diameter nanotubes, but for larger diameter, such tungsten wire and shrinkage could be achieved while still producing a singlelayer carbon nanotube wall.

From the central spinning axis of the the centrifuge there will be strung heat resistant tungsten nanowire that will run through the holes in the wall and into long tubes of small diameter, essentially forming a nanotube mold for cabon to be forced in by the centrifugal force.

Through the tungsten wire there will run an electric current to heat it up and help weld the compressed carbon atoms.
kris2lee
1 / 5 (1) Jun 18, 2012
A solid volume of just 1 cubic meter of nanotubes would contain, laid end-to-end, more than a quadrillion kilometers of nanotubes (10 to the 15th kilometers - which is about 100 light years worth...either I just miscalculated three times in a row or it really is that much).


I'm getting from 10 to 1000 light years depenging on what you take for hair diameter.

When we take 1 cm for the average diameter of the space elevator cable then about 3.5 cubic meters are required by my calcualtions.
hikenboot
not rated yet Jun 18, 2012
instead of a space elevator create a 100 mile wide super conducting launching tube or however long the launch tube would have to be...
hikenboot
not rated yet Jun 18, 2012
ok try number two make a long launch tube maybe a 100 miles long that uses superconducting rails to launch vehicles into space forget the elevator and the nano tubes.
hb_
not rated yet Jun 19, 2012
Doesn't anybody see that this really no news at all? First of all, it's only a CALCULATION. They havn't even shown that they can improve the growth in a lab situation (let alone in an industrial production).

Now, calculations have their limitations. Do they really take into consideration contaminants? They are sure to be present.. Second, is it really realistic to assume perfect conditions throughout the reaction chamber? No heat gradient, no concentration gradient..?

Third, scientist have been using iron as a catalyst for nanotube growth for over a decade, and no one has come close to perfect growth. The norm is a large variation in diameter, chirality and length. And, I allmost forgot, nobody has been anywhere near a meter for a single nanotube!
Riks
not rated yet Jun 20, 2012
Whenever I see headlines including stuff like "50,000 times thinner than a human hair", I suspect that pernicious influence of innumeracy.
Whose hair? From what part of her body? What color? Old engineers will remember the famous RCH.
And that's before harumphing on "times thinner".
Just report distances in meters, with appropriate scaling.
Kafpauzo
5 / 5 (1) Jun 24, 2012
Whenever I see headlines including stuff like "50,000 times thinner than a human hair", I suspect that pernicious influence of innumeracy. [...] Just report distances in meters, with appropriate scaling.


Why can't you use standard journalistic units? The conversion is quite simple: A nanotube is 20 microhumanhairs wide, which amounts to approximately 30 picoschoolbusses.

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